We developed an in vitro system that synthesizes circular single-stranded DNA of phi chi-174. DNA synthesis in the system depends upon the presence of phi chi-174 specific proteins and the process of single-stranded DNA is similar to that found in vivo. We propose that this single-stranded DNA synthesizing system can be coupled with our DNA-directed RNA-dependent protein synthesizing system (transcription and translation coupled system) in order to synthesize phi chi-174 bacteriophage in vitro. BIBLIOGRAPHIC REFERENCES: M. Hayashi, F.K. Fujimura and M. Hayashi (1976). Mapping in vitro messenger RNAs for bacteriophage phi chi-174. Proc. Natl. Acad. Sci., U.S.A. 73:3519. H. Fujisawa and M. Hayashi (1977). Functions of gene C and gene D products of bacteriophage phi chi-174. J. of Virol (Feb. issue).